Lead iodine film

ABSTRACT

A lead iodide photographic film having photosensitivity which is low at room temperature but which can be portionally increased. Precursors of the lead iodide can be coated in separate layers on a film support. A dry developing agent, such as ascorbic acid, and a heat trigger, such as acetamide, can be incorporated to enhance the temperature sensitive photoresponse of the film. The image can be fixed with an alkaline solvent or by plating a more noble metal on the lead, such as copper. A method and mechanism are disclosed for add-an-image exposure and processing.

United States Patent Jacobs et a1;

[ 51 May 9,1972

541 LEAD IODINE FILM [72] inventors: John H. Jacobs, Altadena; Richard A.

. Corrigan, Pasadena, both of Calif.

3,438,776 4/1969 Yudelson ..96/76 3,418,119 6/1954 Schwartz et 211.. ..,96/27 R27,0l2 12/1970 Berman et a1 ..96/48 FOREIGN PATENTS OR APPLICATIONS 1,045,487 10/1966 Great Britain ..96/88 Primary E.\aminerNorman G. Torchin Assistant Examiner-Richard E. Ficht Attorney-Nilsson, Robbins, Wills & Berliner 57 ABSTRACT A lead iodide photographic film having photosensitivity which is low at room temperature but which can be portionally increased. Precursors of the lead iodide can be coated in separate layers on a film support. A dry developing agent, such as ascorbic acid, and a heat trigger, such as acetamide, can be incorporated to enhance the temperature sensitive photoresponse of the film. The image can be fixed with an alkaline solvent or by plating a more noble metal on the lead, such as copper. A method and mechanism are disclosed for add-an-image exposure and processing.

10 Claims, 3 Drawing Figures 1 Field of the Invention The fields of art to which the invention pertains include the fields of light-sensitive elements and photosensitive coating preparation.

2. Description of the Prior Art Particulate lead iodide photosensitive systems have been previously described, for example, in U.S. Pat. Nos. 2,084,420 to Weyde and 2,414,839 to Schoen and in British Pat. Nos. 1,086,384 to Moore and 1,045,487 to Forty. In the patent to Schoen, light-sensitive lead compounds are precipitated onto a support such as a paper sheet or coated thereon from a gelatin solution or emulsion of the light-sensitive lead compounds. The patent to Weyde teaches the aqueous precipitation of lead iodide by reaction of lead nitrate with potassium iodide and subsequent emulsification in gelatin solution to obtain a light-sensitive material. The British patent to Moore also discloses a gelatin emulsion in which lead iodide is precipitated by reaction of lead nitrate and potassium iodide in a dextrin solution and then added to a gelatin solution. The British patent to Forty relates to single crystal photo-sensitivity of lead iodide and describes the behavior of such single crystals with changes in temperature. Although such effects and effects as photoconductivity have been described, the art has not disclosed a practical photosystem utilizing lead iodide. In particular, the art has not disclosed a lead iodide photosystem in which resolution is comparable to silver halide photography and in which the temperature sensitivity of lead iodide crystals is utilized to advantage.

SUMMARY OF THE INVENTION The present invention provides a particulate lead iodide photographic system with unique and advantageous proper- I ties. In particular, films are provided herein utilizing a dispersion of lead iodide crystals and having enhanced temperaturesensitive photoresponse. In accordance with this invention, an add-an-image exposure and processing system is obtainable in which a microfiche or the like can be exposed and processed frame by frame. The film is stable at room temperature and not photosensitive, but when portions of the film are selectively heated to an elevated temperature, or are imbibed with a developing agent, those portions exhibit photosensitivity. Accordingly, one may add an image to a microfiche which has been previously exposed and processed, file the microfiche away with the added image and then at a later time add still another image.

The enhanced temperature-sensitive photoresponse of the films of this invention are obtained by the manner in which the film is prepared. A support is provided and a coating is formed thereon of binder material containing one of the precursors of lead iodide, Le, a lead salt or an iodide. The other precursor reagent is then applied to that coating. The other reagent may be applied by imbibing the coating with a solution thereof whereby lead iodide is uniformly precipitated in fine crystal form in the coating. Alternatively and preferably, a second coating is formed on the first coating of binder material containing the other reagent. More particularly, the other reagent is added to binder material in liquid form which is then coated on the first coating. The first coating can be treated to reduce its permeability by hardening of its surface or by overcoating with a protective interlayer.

To enhance the temperature-sensitive photoresponse of the film, a developing agent in dry form, such as ascorbic acid, can be incorporated in one or both of the layers and can be heat triggered by means of an incorporated polar material which is in mobilized, fused form to act as a solvent in aiding growth of the lead iodide crystals, for example, acetamide. When the film is heated to an elevated temperature, lead iodide crystals of increased size are formed, enhancing its photolytic decomposition to lead. An alkaline solvent can then be applied to the exposed film to dissolve unexposed lead iodide, thereby fixing the image. The image can also be fixedby applying a solution of a salt of metal more noble than lead such as copper, whereby to plate the metal onto the lead.

A mechanism for effecting add-an-image exposure is provided in which a microfiche is secured in alignment with respect to various processing devices. In accordance with this embodiment, means are provided for treating a selected portion only of the secured microfiche to sensitize a frame of the microfiche and means are provided for imagewise exposing the sensitized frame to form an image on that frame. The treating means can comprise a member which heats a selected portion only of the microfiche or can comprise means for imbibing developing agent in such selected portion. Provision can be made for applying fixing material to the frame.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic perspective view of a mechanism for sequential heat sensitizing, exposing and fixing a microfiche;

FIG. 2 is a schematic perspective view of a mechanism for applying developing agents to a selected portion of a microfiche; and

FIG. 3 is a schematic cross-sectional view of a multilayer lead iodide film of this invention.

DETAILED DESCRIPTION As required, detailed illustrative embodiments of the invention are disclosed herein. However, it is to be understood that these embodiments merely exemplify the invention which may take many different forms that are radically different from the specific illustrative embodiments disclosed. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims defining the scope of the invention.

Referring to FIG. 1, there is illustrated a mechanism for the frame-by-frame imaging of a microfiche or the like. The microfiche 10 is illustrated as having a number of frames 12 of information recorded as a result of previous imaging. It is now desired to add an additional frame of information adjacent the last added frame. For example, it may be desired to crossreference certain information contained on a strip of microfilm 14 so that such information may be readily accessible and retrievable under a variety of different classifications. The frames of the microfilm 14 may correspond to the pages of a technical journal, or a series of such journals, whereas the frames 12 on any particular microfiche 10 may relate to a particular subject. In such case, only those frames relating to that subject as found on the microfilm 14 are to be duplicated on the microfiche card 10.

The illustration in FIG. 1 is a schematic representation of three processing stations through which the microfiche 10 passes in order to add an image in accordance with the present invention. The microfiche 10 includes an edge area shown by dashed lines 16 along the top of the film 18 of photosensitive material as hereinafter described. The edge area 16 can contain identifying material or can be appropriately coded for automatic retrieval. The photosensitive film is formulated utilizing lead iodide or precursors thereof in a manner as will be described hereinafter. The nature of the film is such as to be initially substantially insensitive to light but the film can be treated to markedly increased photosensitivity. The film 18 is formulated so that its photoresponse is temperature-sensitive. In particular, the film 18 is photosensitive only after its temperature has been raised to above about C. for a period of time sufficient to ripen the film, as further detailed below.

In the schematic illustration of FIG. 1, a pair of guide rails 20 and 22 are provided through which the microfiche 10 can travel from a ripening station 24, past an exposure station 26, to a fixing station 28. At the ripening station 24, a pair of heated metal plungers 30 and 32 are pressed against the film for about 30 seconds. The metal plungers have contact surfaces 34 and 36 which are heated to about 125 C. and are shaped to define an image frame on the film 18. By such means, a portion of the film 18 is heated as corresponds to the area in which it is desired to place an image, and only that portion is so heated.

The aligned microfiche is then transported from the ripening station 24 to the exposure station 26 where it is pressed by an aperture plate 35 into contact with a platen 36 which is heated to about 100 C. The aperture plate 35 defines a central opening 38 and the microfilm strip 14 is disposed behind the plate 35 in register with the opening 38. The opening 38 is in the shape of a single frame for the microfiche and is in register with the plungers 30 and 32, by means not shown, so as to exactly overlie the presensitized portion of the film 18. A source of light, such as a quartz-iodine incandescent lamp, indicated schematically at 40, is disposed so as to expose the ripened microfiche frame to a frame of the microfilm 14 pressed in contact therewith. Exposure time depends on the sensitivity of the particular microfiche being utilized; with an unsensitized multi-layer lead iodide microfiche as described hereinbelow, exposure time is typically 60 seconds with a 625 watt Sun Sylvania quartz-iodide lamp 40 spaced 1 foot from the aperture plate 35.

Following exposure, the microfiche 10 then passes to the fixing station 28 where a flat shoe 48 sandwiches the imaged area between an agar web 44 and a heated platen 42. The platen 42 is heated to a temperature which is predetermined by the fixing requirements of the emulsion, e.g., 8085 C. for the emulsion utilized in the example. The agar web 44 contains fixing materials as further detailed below and is in the form of an elongated strip about as wide as a frame 12, fed from a feed roller 46 over the shoe 48 and then onto a takeup roller 50. The rollers 46, 48 and 50 are disposed in register with the plungers 30 and 32 and the aperture plate 35, by a mechanism not shown, so that the agar web strip 44 contacts only the frame image at the exposure station 26.

The result of the foregoing operation is a printed frame containing the information that was in the negative microfilm strip 14. The lead iodide microfiche 10 may now be handled in daylight, stored and used in a microfilm viewer until it is desired to add more information from another microfilm strip negative. The operations of sensitizing, or ripening, imaging and fixing are then repeated at another frame position on the film by simply shifting the ripening, exposure and fixing components in register to the next succeeding frame position.

Critical to success of the aforegoing add-an-image photographic method is the provision of a microfiche which can be sensitized frame-by-frame. With the apparatus schematically represented in FIG. 1, the microfiche must have temperaturesensitive photoresponse, i.e., it must be initially relatively insensitive at room temperature, but photosensitive after being subjected to elevated temperatures. Such film can be obtained by effecting a dispersion of lead iodide crystals or precursors thereof in an appropriate binder on a suitable film base. Importantly, the manner in which the dispersion is obtained affects the quality of the film. Lead iodide precipitates which are washed by decantation before adding to a colloid binder tend to coagulate, giving coatings which produce speckled images. Emulsions having small particle size, and hence having potential for high resolution, can be prepared by precipitating the lead iodide in a solution of the binder; but this procedure results in changes in the binder which impair resolution and makes coating difficult. In accordance with the present invention, a suitable film is prepared by forming a coating on a support of binder material containing one of the precursor reagents for the precipitation of lead iodide, and then applying to the coating the other of such reagents. For example, an iodide such as potassium iodide can be dissolved in a solution containing binder material and coated onto a film support. A solution of lead salt such as lead nitrate can then be imbibed into the coating whereupon lead iodide crystals are precipitated as a uniform dispersion throughout the coating. Alternatively, the lead salt can be first coated in a binder with subsequent imbibing of the iodide.

In a preferred method of preparation, constituting a particular embodiment of this invention, the second reagent is applied not by imbibing but in the form of a second coating. For example, after coating a solution of binder and iodide onto the support, a solution of binder and lead salt is applied as a separate coating. During drying, the reagents in the binder layers interact by diffusion to precipitate fine crystals of lead iodide in the vicinity of contact of the layers. Referring to FIG. 3, there is schematically illustrated a lead iodide film prepared in accordance with this last embodiment. The film includes a transparentfilm base 52 supporting a layer of binder 54 containing potassium iodide which, in turn supports a layer 56 of binder containing lead nitrate. A dispersion of fine crystals 58 is formed at the interface 60 between the bottom and top layers. In a similar manner, a lead iodide film can be prepared by first coating a solution of binder and lead salt onto the supportv 52 followed by a coating from a solution of binder and iodide. Films such as the foregoing are referred to throughout the specification as "lead iodide film" although it shouldbe recognized that it is possible that the coating may be accomplished in such a manner as to avoid the formation of lead iodide until some subsequent step, the components being present only in their unreacted form.

With respect to the nature of the components, any reagents can be utilized which will react to precipitate crystals of lead iodide. Thus, one can generally utilize any lead salt more soluable than lead iodide and which is not incompatible in the system, such as lead nitrate, lead chloride, lead chlorate, lead peroxy disulfate and lead fluorosilicate. lodides that can be utilized should also be more soluble than lead iodide and examples stannous iodide, cadmium iodide, zinc iodide, the alkaline and alkaline earth iodides such as potassium iodide, sodium iodide, lithium iodide, magnesium iodide, calcium iodide, strontium iodide, and the like and mixtures thereof. The combination of lead nitrate and potassium iodide yields particularly efficacious results. The reagents can be utilized in equivalent weight amounts, although somewhat better response can be obtained with a slight excess of lead salt. This excess can be obtained by increasing the concentration in, or thickness of, that layer. When using lead nitrate and potassium iodide, there is an approximate coincidence of equivalent weight so that equal volumes of equal concentrations are stoichiometrically equivalent.

Lead iodide which has been precipitated in the binder by any of the foregoing methods generally has a pale lemon yellow color and a particle size of 0.5-2.0 microns. in this form, photosensitivity is extremely low. However, in accordance with another embodiment of this invention, the dispersion is heated so as to increase the particle size as well as the photosensitivity. This change is accompanied by a color change to a deeper orange yellow. The change is facilitated if the coating or coatings contain water which has not been entirely dried out, or if it contains a heat trigger, i.e., a material which holds water in the layer or which is a water substitute to yield a solvent for lead iodide crystal growth. Preferred materials are solid at room temperature but fuse at elevated temperatures to yield a solvent for growth of the lead iodide crystals. When utilizing a heat trigger, the emulsion may be ripened, i.e., sensitized, by heating to an elevated temperature. The effect is very local, particularly when a multi-layered structure is utilized as illustrated in FIG. 3, and does not occur beyond the boundaries of the applied heat. Accordingly, such material may be sensitized one frame at a time by a simple heating process as described with respect to FIG. 1.

As a heat trigger one can incorporate in one or both of the layers (if a multi-layered structure is utilized), a polar solid which is fusible to form a solvent for the growth of lead iodide crystals. Such heat triggers are generally nonaqueous solids containing electron withdrawing groups, for example, -CO- and -SO,.-, have high dielectric constants and provide in the fused state mobility for the lead iodide whereby the crystals of lead iodide can increase in size. Such materials are described in detail in U.S. Pat. No. 3,438,776 to J. S. Yudelson, the disclosure of which is incorporated herein by reference. In particular, the heat triggers may be represented by the following formulas:

(I) XCONRR' (II) YSO NH (III) II RC CO IIeCO (IV) ZCONRR I)": XCON W wherein X is H-, alkyl groups containing 1 to 4 carbon atoms, R'NH-, R"O-, or -(Cl-I )-CONRR; n is l to 4; R is H-, -CI-I or HOCI-I CI-I R is H-, or CI-I R" is a lower alkyl group of 1 to 4 carbon atoms; 2 and R' together represent a divalent radical which is -(CH ),,CO-, or -(CH a is 2 to 3, b is 3 to 5; Y is an alkyl group of l to 4 carbon atoms, NH R"O-, or aryl; and W represents remaining members of a hetrocyclic ring.

These compounds preferably have melting points below about 140 C. Suitable examples include: acetamide, formamide, propionamide, valeramide, N-methyl-acetamide, N,N- dimethyl-acetamide, N-(2-hydroxyethyl)acetamide, N,N'- bis(2-hydroxyethyl) malonamide, N,N'-bis( 2-hydroxyethyl)adipamide, N,N'-bis(2-hydroxyethyl)N,N'-dimethylsuccinamide, succinimide, glutarimide, Z-pyrrolidinone, 2-oxohexamethylenimine, ethyl carbamate, urea, N-methylurea, N,N-dimethylurea, sulfamide, methyl sulfamate, ethyl sulfamate, methylsulfonamide, ethylsulfonamide, p-toluenesulfonamide, ethylene carbonate, and propylene carbonate.

These materials may be used alone or in combination with other materials, or with each other, either as mixtures, complexes or eutectics. Some compounds included in the generic formulas are liquid at room temperature, but when combined with other compounds covered by the generic formula, or with many other types of compounds, will form mixtures which are solid at room temperature. Also some compounds with desirable solvent properties but with melting points too high for practical operating conditions can be incorporated into nonaqueous solvents by mixture with other substances which will lower their melting point. Such mixtures are set forth in the above noted U.S. Pat. No. 3,438,776.

When utilizing a plurality of layers, some ripening or sensitivity may occur upon long periods of storage. Storage ripening may be reduced by utilizing an interlayer between the two reagent layers. Such an interlayer may consist merely of binder material which is separately coated between the layers, acting as a spacing, or it may contain fusible material of the type described immediately above, e.g., acetamide. Altematively, or additionally, the surface of the first-coated reagent layer may be hardened by a variety of treatments. For example, a dilute solution containing borate ions, such as from boric acid or borax, e.g., 0.1-5.0 weight percent may be applied to the layer whereupon the surface thereof hardens upon drying and becomes relatively impervious to diifusion at room temperature. Other materials which can be utilized to provide a hardened surface to the first reagent layer include formaldehyde, other aldehydes and derivatives of aldehydes such as glyoxal or Z-ethyl butyraldehyde, dibasic organic acids such as oxalic acid, malonic acid and succinic acid and other materials as described in Water-Soluble Resins by R. L. Davidson and M. Sittig, Reinhold, 1962, pages 106-107, incorporated herein by reference.

When a lead image is formed from lead iodide by photolysis, iodine is a byproduct. For improved sensitivity, as well as to reduce fading of the image due to recombination, the iodine can be removed by including a reducing agent in the composi tion. Materials known as silver halide photographic developing agents are effective as reducing agents, for example, ascorbic acid, Phenidone B, hydroquinone, Metol,o-chlorydroquinone, glycine, catechol, toluhydroquinone, o-bromohydroquinone, 4-phenylcatechol, Amidol, 4-t-butylcatechol, pyrogallol, 4-n-butylpyrogallol, nordihydroguiauretic acid, 4,5- dibromocatechol, 3,5,6-tribromo-4-phenylcatechol, l-phenyl- 3-(N-n-hexylcarboxamine)-4-[P-B-hydroquinoylethyl)-phenylazo]-5-pyrazolone, and the like and mixtures thereof.

The foregoing materials are conventional silver halide developing agents, and any such agent can be utilized in this invention as a reducing agent for the lead iodide. Of the foregoing compounds, ascorbic acid is greatly preferred. Since ascorbic acid is a polyol, it also acts as a humectant and plasticizer, improving the mechanical behavior of the material and permitting the inclusion in some binders, such as polyvinyl alcohol, of dissolved salts which would otherwise crystallize out. Accordingly, the ascorbic acid not only acts as a reducing agent but also aids in promoting ripening of the lead iodide. The reducing agent may be added to any of the layers or may be coated in an adjacent layer, becoming active when the material is exposed on the hot platen and preventing accumulation of the by-products of photolysis.

Alternatively, the reducing agent can be applied to the film in the form of a solution, in which case sensitization at room temperature occurs. Referring to FIG. 2, a microfiche card 62 is illustrated including film 64 of lead iodide coated on a film base but without a reducing agent. The lead iodide has been preripened by heating the film to effect growth of the lead iodide crystals, but it will not photolyze significantly at room temperature under ambient light because of the absence of a reducing agent. Just prior to exposure, a web 66 moistened with a solution of reducing agent, e.g., in methanol, is fed from a feed roll 68, pressed against the film 64 by a pressure shoe 70 and taken up bya take-up roller 72. By such means, the reducing agent is applied to the selected area only. Altematively, the reducing agent may be contained together with a heat-trigger solvent, such as acetamide, or other polar solid as described above, and applied to the film 64 concurrently with heating of the film 64 by means of a heated platen (not shown) or the like, whereupon reducing agent is transferred from the web onto the film 64 to sensitize the lead iodide. In this embodiment a promoter or promoter precursor can be utilized in conjunction with the reducing agent to further increase room temperature photolysis. Such materials are known to the art and are alkaline or yield alkalies on heating. They include compounds such as ammonium formate, 2-amino-2 methyl-l 3-propanediol, borax, sodium carbonate, potassium oxalate, sodium malonate, sodium succinate and the like, or mixtures thereof. Other suitable materials can be found in U.S. Pat. No. 3,041,170 to G. M. Haist and J. R. King, incorporated herein by reference.

A lead iodide emulsion coated without added reducing agents is stable at room temperature in ambient light, but after a reducing agent as described has been added, slow room light photolysis takes place. Because of this, and also because the unexposed areas of the coating are bright yellow and not transparent, it is desirable to fix the image so that the unexposed components are dissolved out while the metal image remains. In accordance with another embodiment of this invention, fixing is accomplished by applying an alkaline solvent to the film to dissolve unexposed lead iodide or precursor reagents. Suitable alkaline solvents are aqueous solutions of caustic alkali such as sodium hydroxide, potassium hydroxide, ammonium hydroxide or ammonium carbonate; basic organic solvents such as 2-methoxyethylamine and monoethanolamine, and other aliphatic amines; or basic solutions of an amine and acetic acid, e.g., at pH 9-12; or salts of organic acids such as ammonium acetate, sodium acetate, sodium citrate, sodium valerate and the like, or mixtures thereof. Aqueous solutions containing from about 5 to about 50 weight percent of alkaline material can be utilized by imbibing the solution into the coated layer or layers, or by applying it from a pad moistened with solution or in the form of a web as illustrated at the fixing station 28 in FIG. 1. Methods of preparing such webs from gelatin, agar, or the like, are well known to the art. The fixing web may be dimensioned so that it can be applied either to a selected small portion only of the lead iodide film, as illustrated in FIG. 1, or to a full size fiche if desired. Although any of the foregoing materials can be utilized, sodium acetate has been found to be particularly advantageous. The utilization of the fixer in the form of a web is also found to be more advantageous than imbibing with solutions, not only for mechanical convenience but for image stability in that fading is generally found to be minimized with web fixing.

In accordance with another embodiment of this invention, fading of the lead image is prevented by plating on the lead a metal which is more resistant to oxidation. This can be accomplished by applying to the imaged film a solution of a salt of metal more noble than lead, e.g., as the halide or sulphate of such metal. Examples of such metal include copper, mercury, silver, gold and palladium. Copper is particularly advantageous since its common salts are readily available and it is sufficiently more electronegative than lead to readily displacelead from the surface of the image. The resulting copper plated image is extremely durable and suitable for archival copies. The copper, or other metal, salt, may be applied by adding it to a fixing solution as previously described and amounts ranging from about 0.5 to about 10.0 weight percent are generally satisfactory. Complexing ions, such as ammonium, must be present to prevent precipitation of copper ions. Ammonium acetate is found to be particularly suitable for use in conjunction with a copper salt.

In order to facilitate coating and subsequent treatment, the binder for the present photosystem is preferably a hydrophilic colloid. Other desirable properties include the ability to withstand elevated temperatures, e.g., 100 200 C., for short periods, and permeability to aqueous solution used to effect post-treatment of the image without softening to the extent that the coating becomes damaged or leaves the support. Accordingly, in accordance with another embodiment of this invention, the binder may be of any of a number of materials such as proteins (i.e., casein, gelatin, zein, thiolated gelatin), alginate, gums, starch derivatives and the like materials which are generally considered to be natural, or derivatives of natural, film-forming materials. In addition, synthetic water-soluble-film formers may also be used to particular advantage in the practice of the instant invention and such materials include polyvinyl alcohol, commercially available water-soluble polyacrylics (e.g., water-soluble polyacrylic acid salts having substantially the molecular weight and water compatibility of polyvinyl alcohol), various commercially available amine or amide aldehyde resins (e.g., A-stage and B-stage urea-formaldehyde, thiourea-formaldehyde, dicyandiamide-formaldehyde, melamine-formaldehyde, benzoguanamine-formaldehyde, and the like resins prepared in the presence of slight excess of borax or similar alkaline agent to retard condensation during emulsification and heating procedures, but permitting later curing of the resins to form a film), and the like. In addition to the essentially synthetic water-soluble resins capable of forming self-sustaining films and maintaining the dispersed phase of the invention in non-agglomerated form, there are a number of cellulose derivative film formers, which include the various water-soluble cellulose ethers, carboxymethylcellulose, hydroxyethylcellulose, hydroxypropylmethylcellulose, and the like. In addition, any compatible mixture of the above described materials may be utilized.

Experiments have indicated that polyvinyl alcohol confers greater photographic sensitivity than hydroxyethylcellulose which in turn confers greater sensitivity than gelatin. However, gelatin shows higher protective action than the other materials in that it is easier with gelatin to make smooth emulsions with included reducing agents. Polyvinyl alcohol is a preferred material in view of its ability to effect high-sensitivity Binder solutions can be coated onto the support by any prior art method, such as by the use of Byrd coating bars. The

amount of reagent in a layer is determined by the coated thickness as well as by the reagent's concentration in the coating composition. Generally, the thicker the coating the better the photoresponse of the film in terms'of density difference between exposed and unexposed portions. In usual applications, the upper limit for thickness is determined, not by density criteria, but by a decrease in resolution. Such resolution decrease may be due to light scattering within the layer during exposure and/or to the depth of the image exceeding the depth of focus of the optical system in which it is used. Accordingly, the upper thickness limit of the coat is determined by factors which depend upon the application in which the material is to be used. Generally, the layers can each be from about 0.0001

to about 0.003 inch thick in the dry state.

With respect to the film base, any suitable material can be used which has general application in the art, a transparent base being used for the preparation of microfiche. Particularly suitable bases are cellulosetriacetate, Mylar or Cronar (commercial polyethylene terephthaletes) and Kodacel (a commercial cellulose acetate).

The following examples will illustrate formulation of the lead iodide films of this invention.

EXAMPLE 1 A solution is formed by admixing the following ingredients with 25 milliliters of an 8 percent aqueous solution of polyvinyl alcohol (duPont Elvanol grade 72-60).

ascorbic acid 5 gm.

2 molar potassium iodide solution 4 ml. acetamide 5 gm. Triton-X-lOO, 25%

in isopropyl alcohol 4 drops acetamide 5 gm. Triton-X400, 25% in isopropyl alcohol 4 drops Water is added to make 50 milliliters and the solution is .coated as above over the first coating and dried for l0-15 minutes with a hot air gun. I

A third solution is formulated by admixing the following ingredients with 25 milliliters of an 8 percent aqueous solution of polyvinyl alcohol (duPont Elvanol" grade 72-60 l molar lead nitrate aqueous solution 4 ml; acetamide 5 gm. Triton-X-IOO, 25%

in isopropyl alcohol 4 drops Water is added to make 50 milliliters and the solution is coated as above on the second coating.

The resulting film can be handled at room temperature in ambient light without photolysis. A small section of the film is heated in the dark to about 135 C. for 30 seconds, for example, by the use of metal plungers as in FIG. 1, to thereby define a frame" of photosensitivity. The film is then moved to an exposure station where it is pressed with its back in contact with a platen heated to about 100 C., pressure being applied by a transparent material, such as glass or quartz which also covers a negative to be printed. The film is exposed through the negative by means of a 625 watt Sylvania quartz-iodine Sun Gun for 60 seconds at 1 foot. I

An agar fixing sheet is prepared by admixing the following ingredients at about -l00 C.

60% aqueous solution of sodium acetate 50 ml. aqueous solution of agar 50 ml. Triton-X- l 00, 25%

in isopropyl alcohol 4 drops The foregoing formulation is dip-coated hot onto a baryta photobase paper at feet per minute. The coating sets on cooling and the web is rolled up without further drying and stored in a polyethylene bag until used.

The foregoing web is applied in the manner depicted at the fixing station 28 in H6. 1. The result of these operations is a printed frame containing the information that was in the negative. The lead iodide sheet may now be handled in daylight and used in a microfilm viewer until it is desired to put more information onto it from another microfilm negative. The operations of heat sensitizing or ripening, exposing and fixing are then repeated at another frame position on the film.

EXAMPLE 2 An agar web is prepared by admixing solutions A, B and C as follows:

(A) Ammonium acetate 12 gm. Cupric nitrate (3H O) 6 gm. Sodium hydroxide 3 gm. Water to make 50 ml.

(B) 5% aqueous solution of agar 50 ml.

(C) Triton-X400, 25%

in isopropyl alcohol 4 drops The mixture is coated as in Example 1. A lead iodide film is formulated, coated, sensitized and exposed as in Example l, but is fixed with the foregoing agar web. The resulting image is found to be overcoated with a very thin layer of copper.

EXAMPLE 3 A binder solution of potassium iodide was prepared by admixing the following ingredients with 80 milliliters of a 6 percent aqueous solution of polyvinyl alcohol (duPont Elvanol" grade 7260).

ascorbic acid 5 gm. Triton-X- 100, 25%

isopropyl solution 2 drops 2 molar aqueous solution of potassium iodide 8 ml. water 10 ml.

The formulation was coated on a commercial subbed polyester transparent base made by Bexford, Ltd. of England, and dried A binder solution of lead nitrate was obtained by admixing the following ingredients with 80 milliliters of a 6 percent aqueous solution of polyvinyl alcohol (duPont Elvanol" grade 72-60).

Triton-X-l00, 25%

in isopropyl alcohol 2 drops 1 molar aqueous solution of lead nitrate 8 ml. water 10 ml.

The lead nitrate binder solution was coated directly onto the dried layer of potassium iodide and dried. Each of the foregoing layers were coated with a Byrd coating bar to 0.006 inch thickness wet film.

The coated film was ripened by contact with a hot platen for 30 seconds at 135 C. and changed in color from lemon yellow to orange yellow. The ripened film was exposed behind a high contrast negative to a 625 watt Sylvania quartz-iodine Sun Gun for 60 seconds at 1 foot.

The background density and the maximum density'of the sample was measured on an electronic densitometer. Since the samples were unfixed, a red filter was utilized in the densitometer beam in order to avoid further photolysis. The difference between the maximum density and the background density can be taken as a relative measure of response of the sample. This difference was found to be 0.81.

EXAMPLE 4 The procedure of Example 3 was repeated except the the lead iodide layer was coated first, followed by the potassium iodide layer. The difference in maximum density and background density was found to be 0.75.

EXAMPLE 5 The procedure of Example 3 was repeated except that in each binder solution, 14 ml. of lead iodide precursor were used instead of the 8 ml. used in Example 3. The difference between the maximum density and the background density was found to be 0.92, illustrating the effect of increased density with greater concentration of reagents.

EXAMPLE 6 The procedure of Example 3 was repeated except that 4 ml. of lead iodide precursor were used in each binder solution in place of the 8 ml. used in Example 3. The difference between the maximum density and background density was found to be only 0.50, illustrating the effect of a decrease in density with a decrease in concentration of the reagents.

EXAMPLE 7 The procedure of Example 3 was repeated except that the layers were each coated to a wet thickness of only 0.0015 inch. The difference between maximum density and background density was found to be only 0.44, illustrating the effect of decrease in density with decrease in thickness of the coatings.

Resolutions obtainable in each of the foregoing formulations are comparable to that obtainable with silver halide. A target pattern containing 256 lines per millimeter can be reproduced very sharply with some of the formulations, indicating that the resolution capability of lead iodide layers can be substantially higher than this level.

We claim:

1. A film of low or no photosensitivity at room temperature but which can be treated to impart or increase sensitivity to light, comprising:

a support;

a first layer of hydrophilic, water-permeable binder material and a lead salt more soluble therein than lead iodide;

a second layer, contiguous to said first layer, of hydrophilic, water-permeable binder material and an iodide more soluble therein than lead iodide; and

crystals of lead iodide, formed as a result of interaction of said lead salt and said iodide, uniformly dispersed in the vicinity of contact of said first and second layers.

2. The invention according to claim 1 including an agent capable of reducing lead iodide upon photoactivation of said lead iodide, said agent being in dry form in one or both of said layers.

3. The invention according to claim 2 in which said agent comprises ascorbic acid.

4. The invention according to claim 1 including, in one or both of said layers, a polar material which is mobilized at elevated temperatures to act as a solvent for effecting growth of lead iodide crystals.

5. The invention according to claim 1 in which said lead salt is lead nitrate and said iodide is potassium iodide.

6. The invention according to claim 4 in which said polar solid comprises acetamide.

7. A method for obtaining a lead image on film comprising:

applying a layer of hydrophilic, water-permeable binder material to a support, said layer containing one of a lead ing an agent capable of reducing lead iodide upon photoactivation of said lead iodide, said agent being in dry form with said dispersion.

9. The invention according to claim 7 including incorporating with said dispersion a polar material which is mobilized at elevated temperatures to act as a solvent for effecting growth of said lead iodide crystals.

10. The invention according to claim 9 in which said polar material is acetamide.

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2. The invention according to claim 1 including an agent capable of reducing lead iodide upon photoactivation of said lead iodide, said agent being in dry form in one or both of said layers.
 3. The invention according to claim 2 in which said agent comprises ascorbic acid.
 4. The invention according to claim 1 including, in one or both of said layers, a polar material which is mobilized at elevated temperatures to act as a solvent for effecting growth of lead iodide crystals.
 5. The invention according to claim 1 in which said lead salt is lead nitrate and said iodide is potassium iodide.
 6. The invention according to claim 4 in which said polar solid comprises acetamide.
 7. A method for obtaining a lead image on film comprising: applying a layer of hydrophilic, water-permeable binder material to a support, said layer containing one of a lead salt and iodide more soluble therein than lead iodide, and applying the other of said lead salt and iodide to said layer whereby to effect a uniform dispersion of crystals of lead iodide; heating said dispersion to an elevated temperature whereby to form crystals of lead iodide of increased size and photosensitivity; image-wise exposing said heated dispersion to actinic light to obtain an image; and processing said exposed dispersion to stabilize said image.
 8. The invention according to claim 7 including incorporating an agent capable of reducing lead iodide upon photoactivation of said lead iodide, said agent being in dry form with said dispersion.
 9. The invention according to claim 7 including incorporating with said dispersion a polar material which is mobilized at elevated temperatures to act as a solvent for effecting growth of said lead iodide crystals.
 10. The invention according to claim 9 in which said polar material is acetamide. 